This invention relates to loudspeakers, and more particularly to a loudspeaker having improved damping means for removing unwanted higher frequency sounds. Even more particularly, this invention relates to an improved loudspeaker containing a fibrous damping material which considerably reduces undesirable Q losses as compared to damping materials heretofore employed.
Conventional loudspeaker housings include the closed and vented varieties. The closed type of housing has no separate opening therein apart from the opening in which the speaker or driver is mounted. Vented housings have, in addition to the opening which houses the speaker, one or more openings which communciate with the exterior of the housing. Usually this type of housing or enclosure has an open tunnel or port which allows the passage of air in and out of the housing. Still another type of housing may include passive radiators in one or more openings in addition to the opening in which the speaker is mounted.
Still another form of speaker housing is known as the transmission line type, and comprises a housing containing a long pipe or labyrinth open at one end on the exterior of the housing, and communicating at its opposite end with the speaker or driver. The length of the labyrinth typically is made about 25% of the of the resonant wave length of the driver or speaker; and the pipe is folded o baffled back and forth intermediate its ends in a labyrinth manner so that the overall configuration of the housing will result in a practical shape.
As a general rule all such speaker housings or cabinets are designed to manipulate the resonant peak sound waves inherent in the moving coils of the associated loudspeakers or drivers.
In the past it has been conventional to employ in housings of the type noted above some form of damping or stuffing material for the damping of undesired sound waves. In some instances the damping material is simply in the form of a lining inside the housing; and in other cases the damping material is stuffed in the housing completely to block off certain portions thereof, such as for example portions of the labyrinth formed in a transmission line housing. Typical materials used in past have been wool, fiberglass and polyester fibers. Foam materials have also been used.
The primary reasons for utilzing damping materials in such housings are twofold. First, the damping material is used to absorb unwanted higher frequency sounds, such as internal reflections and standing wave resonances between walls of the cabinet, and to the extent that the lower frequencies are attenuated to a much lower degree, the damping material therefore acts as a desirable low-pass filter. Secondly, the damping materials are used to reduce the necessary cabinet volume. In transmission line or labyrinth type enclosures, the damping material also acts to reduce the speed of sound, thereby reducing the necessary line length and enclosure dimension. In enclosed housings, or vented housings (including housings of the type containing passive radiators), the isothermal characteristics of the damping materials are used effectively to increase the box compliance or size.
Damping materials, however, also have certain limitations. For example, the use of such materials causes absorptive losses, which normally tend to increase in correspondence with the increase in the quantity of damping material employed. In the vented form of enclosure these losses decrease the enclosure "Q", which for purposes of this application will be understood to describe resonant magnification in speaker housings. (See The Loud Speaker Design Cookbook by Vance Dickason, Third Ed.) As a consequence, the presence of the damping material contributes to a reduction in the efficiency of the speaker housing or box. For this reason most vented enclosures use a minimal amount of damping material, such as for example simply by coating the inner walls of the enclosure with the damping material.
A primary purpose of this invention, therefore, is to improve the efficiency of a speaker housing by increasing the quantity of damping material in the housing without significantly increasing its absortive losses.
A more specific object of this invention is to provide for speaker housings of the type described a damping material which can be used in large quantities without causing excessive absorptive losses, thereby permitting the use of smaller and more efficient housings.
Other objects of the invention will be apparent hereinafter from the specification and the recital of the appended claims, particularly when read in conjunction with the accompanying drawing